Evaluating the Measurement Accuracy of Surface Water Flows and Accumulated Volumes

Many water users have been strongly encouraged to reduce the amount of their diversions through improved water management practices. To identify opportunities for improving a user's water management practices, an estimate of the accuracy of the flow rates and accumulated volumes withdrawn by the user must be made. Typically, water users measure flow at a given interval (e.g., daily) and then integrate them over time to estimate the volume measurement. In the past, estimates of the uncertainties for individual flow measurements were obtained using standard statistical methods; however, estimates of the error for the accumulated volumes have been difficult. In this study, a procedure is outlined that identifies the random and systematic error components for individual flow measurements. These estimates of the individual error components can be combined to yield an estimate of the uncertainty for an individual flow rate measurement. The uncertainty for the accumulated volume that passes through a given site is then estimated by using 1) the uncertainty associated with the individual flow measurements, 2) the errors associated with integration over time, 3) and the errors introduced into the system during the time between individual flow measurements. These uncertainty estimates can then be used to improve water users' flow measurement strategies.